In recent years, various methods have been studied to form an electric conductor by forming a pattern employing a fine particle dispersion having metal-containing fine particles dispersed in a liquid, followed by a heat treatment (hereinafter referred to simply also as “firing”) so that the metal fine particles are mutually sintered. Such methods may, for example, be a method of carrying out formation and repair of a circuit pattern such as a printed wiring, formation of an interlayer wiring in semiconductor packages, and joining of printed wiring boards and electronic components, by means of an ink jet printing method (e.g. Patent Document 1), a method of joining metals which replaces conventional soldering (e.g. Patent Document 2), and a method of forming an electrically conductive metal film capable of replacing a plated film in the field of electronic materials (e.g. Patent Document 3).
The above-described methods employ known nature called surface melting of metal particles (e.g. Non-patent Document 1). It is generally known that the surface melting of metal particles takes place due to abnormal lattice vibration of atoms at the surface of the particles, and that the smaller the particle diameter and the higher the proportion of surface atoms, the more the surface melting temperature decreases. For example, in a case of silver, it is known that bulk silver has a melting point of about 970° C., whereas fine particles (colloid) having diameters of about 10 nm undergo the surface melting at a temperature of about 80° C. Since the surface melting depends on the particle diameter of the metal particles, it takes place even in an association state so long as individual metal fine particles have a predetermined particle diameter, unless particles are completely bound.
In order to disperse metal fine particles in a liquid, it is common to use a dispersant to prevent agglomeration. Here, in order to achieve excellent dispersion of the metal fine particles in an organic solvent, it is necessary to form a great steric hindrance on the surface of the fine particles. However, when a great steric hindrance is to be formed, it tends to be required to have the dispersant highly polymerized or to increase the amount of the dispersant to be added. Further, in a case where an electric conductor is to be formed by employing a fine particle dispersion having metal fine particles dispersed, if the highly polymerized dispersant is used or the amount of the dispersant is increased, the dispersant can not be removed unless firing is carried out at a high temperature, whereby it tends to be difficult to obtain electrical conductivity.
Further, the lower the firing temperature the better, from the viewpoint of problems of the heat resistance of a board in wiring on printed wiring boards or an element is in wiring in semiconductor packages.
Patent Document 1: JP-A-2002-324966
Patent Document 2: JP-A-2002-126869
Patent Document 3: JP-A-2002-334618
Non-patent Document 1: “J. Sol-Gel Science and Technology”, The Netherlands, Kluwer Academic Publishers, 2001, vol. 22, p. 151-166